Eddy Field in the Japan Sea Derived from Satellite Altimetric Data

The Japan Sea is one of the eddy-rich areas in the world. Many researchers have described the variability of the eddy field and its structure in the Tsushima Warm Current region. On the other hand, since there are few data covering the northern part of the Japan Sea, we are not able to understand the detailed variability of the eddy field there. The variation of the eddy field in the Japan Sea is investigated using the temporal fluctuations of sea surface height measured by altimetric data from TOPEX/POSEIDON and ERS-2. Tidal signals are eliminated from the altimetric data on the basis of the results of Morimoto et al. (2000). Distributions of sea surface dynamic height are produced by using the optimal interpolation method every month. The distributions warm and cold eddies that we obtained coincide well with the observed isotherms at 100 m depth measured by the Japan Sea National Fisheries Research Institute and the sea surface temperature measured by satellite. There are areas with high RMS variability of temporal fluctuation of sea surface dynamic height in the Yamato Basin, the Ulleung Basin, east of North Korea, the eastern part of the Yamato Rise, the Tsushima Strait and west of Hokkaido. The characteristics of eddy propagation in the high RMS variability regions are examined using a lag correlation analysis. Seasonal variations in the number of warm and cold eddies are also examined.     

Characteristics of Sea Surface Circulation and Eddy Field in the South China Sea Revealed by Satellite Altimetric Data

Temporal and spatial variations of sea surface circulation in the South China Sea were revealed with use of altimetric data provided by TOPEX/POSEIDON from December 1992 to October 1997. The estimated distribution of sea surface dynamic heights from altimetric data coincide well with the results of observation by Soong et al. (1995) and Chu et al. (1998). The RMS variability of sea surface dynamic height, which is obtained after tidal correction based on Yanagi et al. (1997), is high in the central part of the South China Sea, the Gulf of Tongking, the Sunda Shelf and the Gulf of Thailand. The high RMS variability in the Gulf of Tongking, the Sunda Shelf and the Gulf of Thailand is due to set up and set down of sea water by the East Asian monsoon, which is northeasterly during winter and southwesterly during summer. Also, the high RMS variability in the central part of the South China Sea is due to the variations of basin-wide circulation. The circulations are dominant in the central part of the South China Sea during summer and winter, an anticyclonic circulation during summer and a cyclonic circulation during winter. It is suggested that these circulations are controlled by the East Asian monsoon. Hence, there is an interannual variability of the basin-wide circulation associated with the variation of the East Asian monsoon.     

Variability of Sea Surface Circulation in the Japan Sea

Composite sea surface dynamic heights (CSSDH) are calculated from both sea surface dynamic heights that are derived from altimetric data of ERS-2 and mean sea surface that is calculated by a numerical model. The CSSDH are consistent with sea surface temperature obtained by satellite and observed water temperature. Assuming the geostrophic balance, sea surface current velocities are calculated. It is found that temporal and spatial variations of sea surface circulation are considerably strong. In order to examine the characteristics of temporal and spatial variation of current pattern, EOF analysis is carried out with use of the CSSDH for 3.5 years. The spatial and temporal variations of mode 1 indicate the strength or weakness of sea surface circulation over the entire Japan Sea associated with seasonal variation of volume transport through the Tsushima Strait. The spatial and temporal variations of mode 2 mostly indicate the temporal variation of the second branch of the Tsushima Warm Current and the East Korean Warm Current. It is suggested that this variation is possibly associated with the seasonal variation of volume transport through the west channel of the Tsushima Strait. Variations of mode 3 indicate the interannual variability in the Yamato Basin.     

日本海环流研究综述

日本海作为东北亚地区最大的边缘海,是西北太平洋上的重要海区。由于特殊的地理位置和复杂的地形,使得日本海的环流结构呈现独有特征,如日本海内的亚极地锋现象,复杂多变的涡旋,北部形成的深水团等。概述了日本海环流状况,着重介绍了对马海峡、郁陵海盆环流情形和日本海特征水团;总结了目前仍存在的争议问题,如对马暖流源头、对马暖流空间结构等;指出了目前日本海尚待解决的科学问题,如对马暖流流量的长期变化及其原因、东韩暖流消失现象及其机制、日本海特征水的传播路径及其影响因素、日本海的某些变化产生原因及其与全球变化的响应等。     

渤海东部潮流动力地貌特征

渤海东部发育了典型的潮流动力地貌体系，它由老铁山水道冲刷槽、辽东浅滩沙脊和渤中浅滩沙席三者组成。前者为潮流侵蚀区，后两者为潮流沉积区。老铁山水道表层最大流速可达５ｋｎ，潮流强烈侵蚀海底；辽东浅滩表层最大流速１．３～２．３ｋｎ，Ｍ２分潮椭率绝对值小于０．４，潮流往复性强，形成潮流沙脊；渤中浅滩表层最大流速为１．２～１．６ｋｎ，Ｍ２分期椭率绝对值大于０．４，潮流旋转性强，形成潮流沙席。６号沙脊南端详测区的旁侧声纳镶嵌日显示的微地貌形态有海底沙丘、沙条、彗星尾标志和冲刷沟，表明沙席和沙脊均是现代潮流作用形成的底形。该潮流动力地貌体系中，净沉积物搬运从老铁山水道至两潮流浅滩；在详测区净沉积物搬运从南向北。     

日本海、鄂霍次克海和白令海的古海洋学研究进展

边缘海的存在使大陆和大洋之间的物质和能量交换变得相当复杂。在构造运动和海平面升降的控制下,边缘海和大洋之间时而连通时而隔绝,各种古气候变化信号都在一定程度上被放大。基于近期有关西北太平洋边缘海的古海洋学研究成果,简要概述了日本海、鄂霍次克海、白令海以及北太平洋地区自中新世以来的古气候和古海洋环境演化特征,并认为它们与全球其它地区一样也受控于因地球轨道参数变化引起的太阳辐射率的变化,大尺度的气候变化具有与地球轨道偏心率周期相对应的100ka周期,而41ka的小尺度周期则受地球自转轴斜率变化的控制。一些突发性的气候变化则是由气候不稳定性、海峡的关闭与开启和其它一些地球气候系统的非线性活动所驱动。但同时作为中高纬度边缘海,它们的古海平面、古海水温度、古洋流等古海洋环境因子的变化特征还受到冰盖扩张和退缩、构造运动、冰川性地壳均衡补偿、东亚季风等因素的影响,具有一定的区域特点。     

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean

A numerical study using a 3-D nonhydrostatic model has been applied to baroclinic processes generated by the K ₁ tidal flow in and around the Kuril Straits. The result shows that large-amplitude unsteady lee waves are generated and cause intense diapycnal mixing all along the Kuril Island Chain to levels of a maximum diapycnal diffusivity exceeding 10³ cm²s⁻¹. Significant water transformation by the vigorous mixing in shallow regions produces the distinct density and potential vorticity (PV) fronts along the Island Chain. The pinched-off eddies that arise and move away from the fronts have the ability to transport a large amount of mixed water (∼14 Sv) to the offshore regions, roughly half being directed to the North Pacific. These features are consistent with recent satellite imagery and in-situ observations, suggesting that diapycnal mixing within the vicinity of the Kuril Islands has a greater impact than was previously supposed on the Okhotsk Sea and the North Pacific. To examine this influence of tidal processes at the Kurils on circulations in the neighboring two basins, another numerical experiment was conducted using an ocean general circulation model with inclusion of tidal mixing along the islands, which gives a better representation of the Okhotsk Sea Mode Water than in the case without the tidal mixing. This is mainly attributed to the added effect of a significant upward salt flux into the surface layer due to tidal mixing in the Kuril Straits, which is subsequently transported to the interior region of the Okhotsk Sea. With a saline flux into the surface layer, cooling in winter in the northern part of the Okhotsk Sea can produce heavier water and thus enhance subduction, which is capable of reproducing a realistic Okhotsk Sea Mode Water. The associated low PV flux from the Kuril Straits to the open North Pacific excites the 2nd baroclinic-mode Kelvin and Rossby waves in addition to the 1st mode. Interestingly, the meridional overturning in the North Pacific is strengthened as a result of the dynamical adjustment caused by these waves, leading to a more realistic reproduction of the North Pacific Intermediate Water (NPIW) than in the case without tidal mixing. Accordingly, the joint effect of tidally-induced transport and transformation dominating in the Kuril Straits and subsequent eddy-transport is considered to play an important role in the ventilation of both the Okhotsk Sea and the North Pacific Ocean. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure of the Tsushima warm current in the northeastern Japan Sea

By using Acoustic Doppler Current Profiler (ADCP) measurements with the four round-trips method to remove diurnal/semidiurnal tidal currents, the detailed current structure and volume transport of the Tsushima Warm Current (TWC) along the northwestern Japanese coast in the northeastern Japan Sea were examined in the period September–October 2000. The volume transport of the First Branch of the TWC (FBTWC) east of the Noto Peninsula was estimated as approximately 1.0 Sv (10⁶ m³/s), and the FBTWC continued to flow along the Honshu Island to the south of the Oga Peninsula. To the north of the Oga Peninsula, the Second Branch of Tsushima Warm Current and the eastward current established by the subarctic front were recombined with the FBTWC and the total volume transport increased to 1.9 Sv. The water properties at each ADCP line strongly suggested that most of the upper portion of the TWC with high temperature and low salinity flowed out to the North Pacific as the Tsugaru Warm Current. In the north of the Tsugaru Strait, the volume transport of the northward current was observed to be as almost 1 Sv. However, the component of the TWC water was small (approximately 0.3 Sv).     

卫星高度计数据样本大小对潮汐信息提取的影响

对3颗高度计卫星TOPEX/POSEIDON(TP),Jason-1(J1),Jason-2(J2)自1992—2011年683个重复周期,共18.6年的数据进行分析,得到全球海洋潮汐调和常数,并重点分析了采用不同样本大小的卫星高度计数据对潮汐信息提取的准度和精度所带来的影响。研究结果表明,参与分析的卫星高度计数据观测样本数的增加可以降低其反演潮汐各分潮振幅时的误差。观测时间为18.6年的高度计数据调和分析所得的主要半日分潮与实测比较,其振幅差相比于利用10年数据的计算结果减小约0.5cm;但是由于忽略了卫星更替过程所带来的观测时间差来进行调和分析,将会对计算分析过程中产生的迟角误差造成影响,主要全日分潮的迟角误差增加约2°,而半日潮迟角误差的改变则比较小。本文进一步用理想化实验解释了造成这种迟角计算误差变化的原因,比较了轨道交叉点上,由卫星在升轨和降轨2个轨道上各自的观测数据计算得到的调和常数,发现随着参与分析的高度计观测样本数的增加,调和分析计算潮汐调和常数时的内符精度也会显著提高。利用18.6年数据比利用10年数据进行调和分析时,主要半日潮调和常数的精度提高了约7%。     

Sequential forecasting of the surface and subsurface conditions in the Japan Sea

This study estimates a realistic change of the Japan Sea by assimilating satellite measurements into an eddy-resolving circulation model. Suboptimal but feasible assimilation schemes of approximate filtering and nudging play essential roles in the system. The sequential update of error covariance significantly outperforms the asymptotic covariance in the sequential assimilation due to the irregular sampling patterns from multiple altimeter satellites. The best estimates show an average rms difference of only 1.2°C from the radiometer data, and also explain about half of the sea level variance measured by the altimeter observation. The subsurface conditions associated with the mesoscale variabilities are also improved, especially in the Tsushima Warm Current region. It is demonstrated that the forecast limit strongly depends on variable, depth, and location.     

A Box Model of Glacial-Interglacial Variability in the Japan Sea

The Japan Sea has experienced drastic changes in the last 60 ka: the surface water was colder than the present value by five degrees and extremely freshened (24 ppt) in the last glacial maximum (15 ka), and then it contained Oyashio water for a few thousand years. It is an open question whether the inflow-outflow pattern was entirely reversed, opposite to the present exchange with an inflow through Tsushima Strait and an outflow through Tsugaru Strait. A box model is employed with two boxes representing the northern and the southern half domains in the upper (300-m-thick) layer. The model is driven by atmospheric forcing and inflow through Tsushima Strait and/or Tsugaru Strait. Here, the net transport through Tsushima to Tsugaru is given in the model. A baroclinic component is added to the net transport through each strait. It is the baroclinic components that allow the upper and the lower portions to flow to the opposite directions in the straits, and hence a reversal flow becomes possible against the net transport, under the condition of an extremely freshened Japan Sea. The fresh surface layer in 1814 ka is attributable to a near-shutoff of the inflow due to the low sea level. Shortly after the near-shutoff, the baroclinic transport through Tsugaru Strait yields intrusion of the Oyashio water into the Japan Sea. Thus, it is implied that Oyashio water existed in the Japan Sea a few thousand years after the reopening of Tsugaru Strait, even though the net transport was one-way, similar to the present state.     

Supply of Heat by Tsushima Warm Current in the East Sea (Japan Sea)

A significant surface net heat loss appears around the Kuroshio and the Tsushima Warm Current regions. The area where the surface heat loss occurs should require heat to be supplied by the current to maintain the long-term annual heat balance. Oceanic heat advection in these regions plays an important role in the heat budget. The spatial distribution of the heat supply by the Tsushima Warm Current near the surface was examined by calculating the horizontal heat supply in the surface layer of the East Sea (the Japan Sea) (ESJS), directly from historical sea surface temperature and current data. We have also found a simple estimation of the effective vertical scale of heat supply by the current to compensate net heat loss using the heat supplied by the current in the surface 10 m layer. The heat supplied by the current for the annual heat balance was large in the Korea/Tsushima Strait and along the Japanese Coast, and was small in the northwestern part of the ESJS. The amount of heat supplied by the current was large in the northwestern part and small in the south-eastern part of the ESJS. These features suggest that the heat supplied by the Tsushima Warm Current is restricted to near the surface around the northeastern part and extends to a deeper layer around the southeastern part of the ESJS. This revised version was published online in August 2006 with corrections to the Cover Date.     

日本海中尺度涡旋时空变化特征研究

本文利用1993–2019年基于海表面高度异常的涡旋数据集和高度计数据统计分析了日本海区域中尺度涡旋的大小、极性、生命周期、振幅、传播等表面特征的时空变化规律。27年间,共探测到1 429个涡旋,气旋和反气旋数量基本相当,其中气旋675个,反气旋754个。两种极性涡旋均具有较强的季节变化：秋季较多,冬季次之,春季最少。郁陵盆地、大和盆地等为涡旋多发区域呈现西南–东北向带状分布。其中,南部海域反气旋占优,靠近津轻海峡的北部海域气旋占优。西部和南部受东韩暖流和对马暖流的驱动,涡旋移动方向与流场基本一致,北部涡旋与黎曼寒流以及副极地锋流有关。研究表明,动力学不稳定是涡旋在秋冬季大量产生的重要原因。此外,半封闭盆地、局地流场以及复杂的海气相互作用等都可能会对涡旋的产生和消亡造成一定影响。     

Cooling-Induced Current in the Upper Ocean of the Japan Sea

The most plausible scenarios for seasonal to interannual variabilities and their possible causes are investigated for the Tsushima Current system passing through the Japan Sea. The study is based on the north and south two-box model across the polar front in an idealized upper ocean of the Japan Sea. The boxes are connected by lateral diffusive heat transport and cooled by atmospheric forcing at the annual mean state. The south box, i.e. the Tsushima Current region, only interacts with the outside warmer box in the East China Sea and has an eastward thermal-driven current originating in the outside box. The magnitude of this current depends on the strength of the thermal gradient between the north and south boxes; the inflow of warm waters can therefore be maintained by net heat loss through the sea-surface. I call such a thermal-driven inflow process a "Cooling-Induced Current" system in the present study. Under periodical heat forcing, the perturbation response of the model to water temperature fields and inflow transport were examined. It is shown that the lateral diffusion time across the polar front (over a period of 10 years) is crucial to the interannual modeled response. An analysis of the seasonal heat budget suggests that the heat transported into the Japan Sea from the East China Sea in summer is stored mainly within the Tsushima Current region and contributes to heat loss by the sea-surface cooling in winter.     

Long-Term Variations of Potential Temperature and Dissolved Oxygen of the Japan Sea Proper Water

Observed potential temperatures and concentrations of dissolved oxygen are analyzed to elucidate their variations during the period from 1958 to 1996 at Stn. P (37°43′ N, 134°43′ E) and from 1965 to 1996 at Stn. H (40°30′ N, 137°40′ E) in the Japan Sea. At Stn. P, increases of the potential temperature for the period are found below 800 m depth with the largest value of 0.16 ± 0.09°C per century at 800 m depth. At Stn. H, the potential temperature increased below 500 m depth. The increase rate has the largest value of 0.50 ± 0.18°C per century at 500 m depth and it is 0.30 ± 0.09°C per century at 800 m depth. The concentrations of dissolved oxygen increased around 800 m depth at Stn. P. At Stn. H, they increased above 800 m depth. On the other hand, they decreased below 1200 m depth at both stations. The layer of the dissolved oxygen minimum has deepened in these decades. These features appearing in the distributions of temperature and dissolved oxygen are successively simulated by a vertical one-dimensional advection-diffusion model including consumption of dissolved oxygen and termination of the deep water supply. These results suggest that the supply of the Japan Sea Proper Water into the deep layer, which is cold and rich in dissolved oxygen, has been decreasing for the last four decades. This revised version was published online in August 2006 with corrections to the Cover Date.     

Intermediate Waters in the East/Japan Sea

Properties of the intermediate layer in the East/Japan Sea are examined by using CREAMS data taken mainly in summer of 1995. Vertical profiles of potential temperature, salinity and dissolved oxygen and relationships between these physical and chemical properties show that the dissolved oxygen concentration of 250 μmol/l, roughly corresponding to 0.6°C at the depth of about 400 db, makes a boundary between intermediate and deep waters. Water colder than 0.6°C has a very stable relationship between potential temperature and salinity while salinity of the water warmer than 0.6°C is lower in the western Japan Basin than that in the eastern Japan Basin. The low salinity water with high oxygen corresponds to the East Sea Intermediate Water (ESIW; <34.06 psu, >250 μmol/l and >1.0°C) which was previously identified by Kim and Chung (1984) and the high salinity water with high oxygen found in eastern Japan Basin is named as the High Salinity Intermediate Water (HSIW; >34.07 psu, >250 μmol/l and >0.6°C). Spatial distribution of salinity and acceleration potential on the surface of σ_ϑ = 27.2 kg/m³ shows that the ESIW prevailing in the western Japan Basin is transported eastward by a zonal flow along the polar front near 40°N and a cyclonic gyre in the eastern Japan Basin is closely related to the HSIW. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tidal bedforms in eastern part of the Bohai Sea

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Basin-To-Basin and Year-To-Year Variation of Temperature and Salinity Characteristics in the East Sea (Sea of Japan)

Analysis of CTD data from four CREAMS expeditions carried out in summers of 1993–1996 produces distinct T-S relationships for the western and eastern Japan Basin, the Ulleung Basin and the Yamato Basin. T-S characteristics are mainly determined by salinity as it changes its horizontal pattern in three layers, which are divided by isotherms of 5°C and 1°C; upper warm water, intermediate water and deep cold water. Upper warm water is most saline in the Ulleung Basin and the Yamato Basin. Salinity of intermediate water is the highest in the eastern Japan Basin. Deep cold water has the highest salinity in the Japan Basin. T-S curves in the western Japan Basin are characterized by a salinity jump around 1.2–1.4°C in the T-S plane, which was previously found off the east coast of Korea associated with the East Sea Intermediate Water (Cho and Kim, 1994). T-S curves for the Japan Basin undergo a large year-to-year variation for water warmer than 0.6°C, which occupies upper 400 m. It is postulated that the year-to-year variation in the Japan Basin is caused by convective overturning in winter. This revised version was published online in August 2006 with corrections to the Cover Date.     

Late Quaternary Seasonal Sea-Ice History of the Northeastern Japan Sea

The Late Quaternary sea-ice history of the northeastern Japan Sea is discussed on the basis of the occurrence of dropstones and ice-rafted debris (IRD) in fine sediment cores. IRD was found in all strata except those from the Holocene and oxygen isotope stage 5.5. The largest expansion of sea ice was recognized at the last glacial maximum (LGM; oxygen isotope stage 2), when the southern margin of seasonal sea ice was probably located in the vicinity of the Matsumae Plateau. The margin might occasionally have expanded further southward to off the Oga Peninsula. Sea ice expanded southward from mid-stage 5 to the LGM in response to global cooling, but with much fluctuation. Sea ice remained during deglaciation until around 10 ka, but after 10 ka it retreated northward rapidly in response to global warming and changes in surface water conditions. Greater fluctuations in IRD were found in core GH95-1208 collected from off Rumoi, Hokkaido, Japan. More IRD was found in sediments from late stage 3, late stage 5, and early stage 6. The fluctuations were not concordant with global climate changes (based on the standard oxygen isotope curve), and may have been controlled by regional climate factors such as the strength of the winter monsoon, which is related in turn to high-latitude atmospheric circulation. This revised version was published online in July 2006 with corrections to the Cover Date.